March 26, 2011

Pastoral and farmer populations from the Sahel

In Central Asia, the expansion of the Mongol nomads led to the predominance of a few Y-chromosome lineages of relatively recent vintage. The opposite seems to be true for the African pastoralists examined here: they see to be more diverse in their Y-chromosomes than farmers are, and less so in their mtDNA. This suggests that they were a fairly old group who did not particularly marry farmers' daughters.

Mol Biol Evol (2011) doi: 10.1093/molbev/msr067

Genetic structure of pastoral and farmer populations in the African Sahel

Viktor Černý et al.

Traditional pastoralists survive in but few places in the world. They can still be encountered in the African Sahel, where annual alternations of dry and wet seasons force them to continual mobility. Little is known about the genetic structure of these populations. We present here the population distribution of 312 HVS-I mtDNA and 364 Y-STR haplotypes in both farmer and pastoralist groups from the Lake Chad Basin and the West African Sahel. We show that the majority of pastoral populations (represented in the African Sahel by the Fulani nomads) fail to show significant departure from neutrality for mtDNA as evidenced by Fu's Fs statistics, and exhibit lower levels of intra-population diversity measures for mtDNA when contrasted with farmers. These differences were not observed for the Y chromosome. Furthermore, AMOVA analyses and population distributions of the mtDNA haplotypes show more heterogeneity in the sedentary groups than in the pastoralists. On the other hand, pastoralists retain a signature of a wide phylogenetic distance contributing to their male gene pool, whereas in at least some of the farmer populations a founder effect and/or drift might have led to the presence of a single major lineage. Interestingly, these observations are in contrast with those recorded in Central Asia, where similar comparisons of farmer and pastoral groups have recently been carried out. We can conclude that in Africa there have been no substantial mating exchanges between the Fulani pastoralists coming to the Lake Chad Basin from the West African Sahel and their farmer neighbors. At the same time we suggest that the emergence of pastoralism might be an earlier and/or a demographically more important event than the introduction of sedentary agriculture, at least in this part of Africa.

These findings are in agreement with the autosomal data on Fulanis in Bryc & Henn et al. They are notably divergent from neighboring sedentary groups, which couldn't be explained by a function of geography.

Also of interest are how complete the Fulani sample is geographically, and whether Fulani are closer genetically to other Niger-Congo language speaking farmer populations than non-Niger-Congo langauge speaking pastoralist populations are to Niger-Congo language speaking farmer populations.

It would also be interesting to see how closely the genetic evidence corresponds to the historical and legendary accounts of Fulani origins. For example, my understanding is that the Fulani are relative latecomers to the Lake Chad basin, yet the study seems to indicate that their existence of a coherent cultural unit may predate the Niger-Congo language speaking farmers they are adjacent to in Africa.

If you look at the first figure the 2 diminsional plot, you can see that several lines of relationship exist.

One line includes the San and the Fulani (at either end) indicating that these groups are part of a relationship group with positions on the line indicating relative admixture. A spectrum of relationship if you like.

OK perhaps it would be clearer to say that the San and the Fulani are not related, but they are both parts of a relationship group. They are the poles in the relationship group. With the other groups representing variable admixtures of Fulani and San (or something very like the San).

Mouse, according to that MDS plot there is no relationship between the Fulani and the unadmixed San that they don't share with all the other Sub-Saharan African populations (including the agriculturalists). Their genetic divergence is stark.

If you read what I say, I say they are in a relationship group. They are part of a connected set of related populations. The relationship spans from San to Fulani. But does not include other groups like the Bantu for example. They do not have to genetically connected if they are at poles of the group.

IMO being part of a relationship group is more significant than absolute distance.

In admixtured populations distance is heavily influenced by the admixture composition. So two genetically similar populations that have not been in contact for millenia will appear closer than an admixtured population made up of diverse populations. Or put another way. Mom may be Japanese, Dad may be European. The child appears Uiger or Indian. But un fact the chid is more closely related to the Japanese and Europeans than first appears. And indeed clearly Japanese Mom met European Dad.

I am reading this as significant contact between hunter gathers (San-like) and pastorialists (Fulani).

"If you read what I say, I say they are in a relationship group. "They are part of a connected set of related populations. The relationship spans from San to Fulani. But does not include other groups like the Bantu for example."

I think what that plot shows at most is a relationship between the pre-Bantu East African populations (Hadza, Sandawe and Maasai). Fulani's position is probably due to their connections with both Western Africa and Northern Africa (not necessarily recent, but their heterogeneous position brings to mind at least some recent connections). The unadmixed San have no visible relationship with these populations according to the plot; on the other hand, they are on the same vertical axis with the unadmixed Pygmies, this may point to a relationship between the two.

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